1. Field of the Invention
Aspects of the present invention relate to a charging device having small loop transmission coils to wirelessly charge target devices such as mobile telephones and tablets. More particularly, aspects of the present invention relate to a wireless charging device using small loop transmission coils having different sizes and connected in parallel.
2. Description of the Related Art
Recent years have seen tremendous growth in wireless charging applications, and this trend is expected to continue at least into the near future. Wireless charging is a technology where electromagnetic induction is used to transmit power through air, without the use of power cords or conductors. A wireless charging system includes: i) a charger, i.e., a power transmitter unit (PTU) with a primary coil, and ii) a target device to be charged (charged device), i.e., a power receiver unit (PRU) with a secondary coil. Power in the charger is transferred to the target device to be charged through the electromagnetically coupled primary and secondary coils, and the induced current may be further processed and used to charge the battery of the target device. Energy is transmitted through inductive coupling from the charger to the target device, which may use that energy to charge batteries or as direct operational power for the target device.
Wireless charging is commonly divided into two types. One of the wireless charging types is a magnetic induction type and the other is a magnetic resonance type. Both of these types work on near field technology, i.e., the electromagnetic field dominates the region close to both the PTU and the PRU. The magnetic induction type includes two coils which are very close to each other, generally within a range of a few millimeters to a few centimeters. The magnetic resonance type includes two resonant coils, which are generally separated in a range of a few centimeters to a few meters, operating at the same resonant frequency, are strongly coupled, and thus a high charging efficiency is achievable.
As illustrated in FIG. 1A, a wireless charger 11 includes an induction coil antenna 12. The wireless charger 11 senses the presence of a mobile device (target device) 13 for charging on a flat surface of a charger base 14. The induction coil antenna 12 creates an alternating electromagnetic field from within the wireless charger 11, and a second induction coil (not shown) in the mobile device 13 takes power from the alternating electromagnetic field and converts the power back into electric current to charge a battery in the mobile device 13 or to provide direct operational power to the mobile device 13. However, the design of a coil antenna in existing wireless chargers usually suffers from a non-uniformity of the magnetic field issue, especially at an edge region of the induction coil antenna 12. This issue is problematic since an unevenly distributed magnetic field over the charger base surface 14 greatly impacts the charging efficiency, particularly in a negative way.
In addition, too small of an electromagnetic field is generated at the edges of the induction coil antenna 12, thus preventing the mobile device 13 from being able to collect enough power through its second induction coil depending upon the placement of the mobile device 13 on the charger base surface 14.
To cope with these problems, a design having a small loop coil structure that is placed under a coil antenna has been proposed recently to tackle the unevenly distributed magnetic field problem. The aforementioned design includes multi-small loop transmission coils with common sizes that are placed under a coil antenna. As illustrated in FIG. 1B, a wireless charger 15 includes a coil antenna 16 with a small loop transmission coil 17 structure positioned underneath the coil antenna 16 to provide a higher magnetic field with better uniformity for wireless charging. However, there is a drawback in that the existing small loop coil 17 structure cannot work normally when a phone with a metallic cover and/or case is placed directly on the coil antenna 16. The metallic materials in the phone cover and/or case alter the capacitance of the small loop transmission coil 17 and offsets the resonant frequency. Consequently, the coupling efficiency for wireless charging will then be greatly affected, particularly in a negative way.